Core-wall rebar installation: reinforcing at the boundary columns were prefabricated with verticals and ties in two-story lifts. The distributed horizontal and vertical bars were field installed.Credit: Tipping Structural Engineers

The LEED Platinum SFPUC HQ is the product of a host of sustainable-design strategies such as a Living Machine, wind turbines, solar arrays, low-cement concrete, and much more and is an exemplary model for water conservation, energy performance, and indoor air quality. Moreover, the vertically post-tensioned concrete structure gives the SFPUC superior resilience, the capability to self-center after a large earthquake with only minimal damage resulting and immediate reoccupation possible.

At the project’s genesis, the SFPUC mandated that the new headquarters be designed to essential-use-facility standards: after a large earthquake, it should be easily reparable and immediately reoccupiable. This dictated a standard of seismic performance that would limit structural deformations during a major shock and allow the building to return to its original position, protecting all the building’s systems. Thus the original design called for base; however, owing to budget constraints, that plan had to be shelved in favor of a system of steel moment frames with viscous dampers. Yet the costs still came to about $62 million higher than budget.

At that point—well into construction documents—contractor Webcor brought Tipping in to redesign the structure in concrete. To cost-effectively meet the performance mandate, we redesigned the structure using a post-tensioned concrete shear-wall system with composite link beams. This delivered immediate-reoccupancy performance at a negligible cost over conventional design, using the floor-and-column grid set during design development. In short, our redesign allowed the oft-stalled, eight-year-old project to finally proceed to completion.

The structural system consists of post-tensioned concrete slabs and beams supported by concrete columns and two concrete core walls that sit at either end of the building and provide the lateral resistance. These core walls sit atop a 10-foot-thick mat foundation with micropiles embedded 65 feet down, a combination designed to resist seismic overturning load. The shear walls are reinforced with conventional bonded reinforcing and vertical nonbonded post-tensioning cables. The PT tendons run from the top of each core wall down through a duct installed within the mat foundation and back up to the top of the wall. This vertical post-tensioning provides the strength and elasticity for the system to yield and then return to plumb after a large earthquake. The core walls enclose some of the building’s elevators, stairs, and mechanical shafts. Atop the wall openings, Tipping designed composite link beams formed of a steel-plate jacket containing reinforcement and concrete. These serve as formwork and a shallower, more flexible beam that is easy to build owing to smaller and less congested reinforcement.

Beyond the superior resilience—and a 100-year building lifespan—brought by the new PT concrete core-wall redesign were further benefits like the addition of a thirteenth floor and improved daylighting (owing to the largely exposed concrete ceiling and walls). The system allowed a 50-percent reduction in structural steel, decreasing both wall congestion and construction time. The redesign also meant significant cost savings to the project, which Engineering News Record captures in detail: $4 million in direct structural costs; $1 million in indirect structural costs, including changes to the basement that reduced shoring, excavation, and waterproofing; $3 million on finishes; $11 million on skin; $10 million on the mechanical system, and $3.7 million on the electrical system.

Lastly, we worked with Central Concrete to design low-cement concrete mixes that reduced embodied carbon by an average of 50 percent. The high-strength mixes were tailored for each different application and specified replacing Portland cement by up to 70 percent, using a combination of slag and fly ash. In total, the cement-replacement strategy accounts for a saving of more than 7 million pounds in CO2 emissions.